Martensitic stainless steel for disc brakes

ABSTRACT

A martensitic stainless steel for disc brakes, having high temper softening resistance. The martensitic stainless steel is not seriously softened by maintaining the steel at more than 600° C. The martensitic stainless steel contains less than 0.050 mass % carbon, 1.0 mass % or less silicon, 2.0 mass % or less manganese, 0.04 mass % or less phosphorus, 0.010 mass % or less sulfur, 0.2 mass % or less aluminum, more than 11.5 mass % to 15.0 mass % chromium, 0.5 mass % to 2.0 mass % nickel, more than 0.50 mass % to 4.0 mass % copper, more than 0.08 mass % to 0.6 mass % niobium, and less than 0.09 mass % nitrogen, the remainder being iron and unavoidable impurities.

This is a Division of application Ser. No. 10/546,248 filed Oct. 5,2005, which in turn is a National Phase of Application No.PCT/JP2004/005934 filed Apr. 23, 2004. The disclosure of the priorapplications is hereby incorporated by reference herein in its entirety.

TECHNICAL FIELD

The present invention relates to martensitic stainless steels for brakediscs (hereinafter referred to as discs) included in disc brakes formotorcycles, motorcars, bicycles, and other vehicles. An example of suchdisc brakes is shown in FIG. 1. The present invention particularlyrelates to a martensitic stainless steel for disc brakes which arehardly softened but have an initial proper hardness after the discbrakes are maintained at high temperature for a long time by frictionheat during braking, that is, which are superior in temper softeningresistance (hereinafter referred to as high-temperature softeningresistance).

BACKGROUND ART

Disc brakes for motorcycles and other vehicles have a function ofslowing the rotation of wheels by the friction between discs and pads.The temperature of the discs is greatly increased due to friction heatgenerated during braking. In recent years, in order to achieve high fuelefficiency to protect the Earth's environment, vehicles have beenreduced in weight and the discs have been therefore reduced inthickness. Since a reduction in disc thickness results in a reduction inheat capacity, the temperature of the discs is greatly increased byfriction during braking. Therefore, there is a possibility that thediscs are rapidly worn away because the discs are tempered and softened.

In view of hardness and corrosion resistance, a low-carbon martensiticstainless steel containing 12% chromium and 0.06% carbon on a mass basisis used to manufacture discs of known disc brakes. The stainless steelis usually machined so as to have a predetermined shape, hardened, andthen provided to users. The composition of the stainless steel isdesigned such that the stainless steel has a hardness of 32 to 38 in HRC(Rockwell C hardness determined according to JIS Z 2245).

After a disc brake made of the martensitic stainless steel is heated tohigh temperature, particularly 550° C. or more, by friction heat, thehardness thereof is seriously decreased due to the relief of the strainof the disc brake and the precipitation of carbonitrides. Therefore, thehardness of the disc brake can be reduced to less than its lower limit,that is, less than 32 in HRC. In particular, since the discs have beenreduced in thickness as described above, the discs need to havehigh-temper softening resistance because the discs are heated to morethan 600° C. in some cases.

In order to meet such a need, Japanese Unexamined Patent ApplicationPublication No. 2002-146482 discloses a steel sheet used to improve thedisc warpage caused by an increase in temperature. However, thetemperature disclosed in this document is up to 600° C. and no techniquefor improving temper softening resistance, which is a key to prevent thehardness of heated steel sheets from being reduced, is disclosed in thedocument. Meanwhile, Japanese Unexamined Patent Application PublicationNo. 2001-220654 discloses another steel sheet with high temper softeningresistance. This steel sheet has a hardness of 30 or more in HRC afterthe steel sheet is maintained at 530° C. or more. However, the hardnessof this steel sheet is insufficient because the following steel sheetshave been recently demanded: steel sheets that have high tempersoftening resistance after they are heated to more than 600° C.

It is an object of the present invention to provide a martensiticstainless steel, having high temper softening resistance, for discbrakes. The steel is slightly tempered and softened if the steel ismaintained at more than 600° C.; that is, the steel has a hardness of 32or more or a hardness of 30 or more in HRC after the steel is temperedat 650° C. or 670° C., respectively.

DISCLOSURE OF INVENTION

In order to solve the above problems involved in the known techniques,the inventors have investigated the effect of the composition ofmartensitic stainless steels on the temper softening resistance. As aresult, the inventors have found that such steels can be prevented frombeing softened due to the relief of strain by adjusting the niobiumcontent and the copper content to proper values to form fineprecipitates containing such elements at 500° C. to 700° C. to preventthe movement of dislocation. The inventors have also found that if thesteels are maintained at more than 600° C., the hardness of the steelscan be maintained high by adjusting the nitrogen content and the nickelcontent to proper values to prevent carbide precipitates from beingformed at high temperature to keep the amount of dissolved carbon tomaintain the hardness of martensitic structures.

The present invention, which has been made based on the above findings,provides a martensitic stainless steel for disc brakes. The steelcontains less than 0.050 mass % carbon, 1.0 mass % or less silicon, 2.0mass % or less manganese, 0.04 mass % or less phosphorus, 0.010 mass %or less sulfur, 0.2 mass % or less aluminum, more than 11.5 mass % to15.0 mass % chromium, 0.5 mass % to 2.0 mass % nickel, more than 0.50mass % to 4.0 mass % copper, more than 0.08 mass % to 0.6 mass %niobium, and less than 0.09 mass % nitrogen, the remainder being ironand unavoidable impurities. The carbon content, the nitrogen content,the niobium content, the chromium content, the silicon content, thenickel content, the manganese content, and the copper content satisfythe following inequalities (1) and (2):

0.03≦[C]+[N]−13/93×[Nb]≦0.09   (1)

5×[Cr]+10×[Si]+30×[Nb]−9×[Ni]−5×[Mn]−3×[Cu]−225×[N]−270×[C]≦40   (2).

Furthermore, the present invention provides another martensiticstainless steel for disc brakes. This steel containing less than 0.050mass % carbon, 1.0 mass % or less silicon, 2.0 mass % or less manganese,0.04 mass % or less phosphorus, 0.010 mass % or less sulfur, 0.2 mass %or less aluminum, more than 11.5 mass % to 15.0 mass % chromium, morethan 0.50 mass % to 2.0 mass % nickel, more than 0.50 mass % to 4.0 mass% copper, more than 0.08 mass % to 0.6 mass % niobium, and less than0.09 mass % nitrogen, the remainder being iron and unavoidableimpurities. The carbon content, the nitrogen content, the niobiumcontent, the chromium content, the silicon content, the nickel content,the manganese content, and the copper content satisfy the followinginequalities (1) and (2):

0.03≦[C]+[N]−13/93×[Nb]≦0.09   (1)

5×[Cr]+10×[Si]+30×[Nb]−9×[Ni]−5×[Mn]−3×[Cu]−225×[N]−270×[C]≦40   (2).

The martensitic stainless steels preferably further contain 0.02 mass %to 0.3 mass % vanadium.

The martensitic stainless steels preferably further contain one or moreof the following elements:

-   (1) one or both of 0.02 mass % to 2.0 mass % molybdenum and 0.02% to    2.0 mass % cobalt on a mass basis;-   (2) one or more of 0.02 mass % to 0.3 mass % titanium, 0.02% to 0.3    mass % zirconium, and 0.02 mass % to 0.3 mass % tantalum on a mass    basis; and-   (3) one or both of 0.0005 mass % to 0.0050 mass % boron and 0.0005    mass % to 0.0050 mass % calcium.

The present invention provides a hot-rolled sheet or cold-rolled sheetmade of one of the martensitic stainless steels.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration showing an example of a brake disc, includinga sheet of steel according to the present invention, for motorcycles.

BEST MODE FOR CARRYING OUT THE INVENTION

The martensitic stainless steel according to the present invention hasthe above composition and therefore has high temper softeningresistance. That is, the steel has a hardness of 32 to 38 in HRC afterthe steel is hardened and the steel has a hardness of 32 or more or ahardness of 30 or more in HRC after the steel is tempered at 650° C. or670° C., respectively. Furthermore, the steel has high toughness andcorrosion resistance.

The reason for limiting the composition of the steel having the abovefeatures to the above range will now be described.

Carbon Content of Less than 0.050 Percent by Mass

Carbon as well as nitrogen is an element useful in enhancing thehardness of the steel by hardening. In order to achieve such anadvantage, the steel preferably contains 0.015 percent or more carbon ona mass basis. When the steel is maintained at more than 600° C., carbonbonds to chromium to form coarse Cr₂₃C₆ precipitates; hence, carbon isuseless in enhancing the hardness and deteriorates the corrosionresistance because carbon creates rust. An excessive increase in carboncontent leads to a reduction in toughness. Therefore, it is necessary tolimit the carbon content to less than 0.050 percent by mass. Inparticular, in order to enhance the toughness and the corrosionresistance, the carbon content is preferably less than 0.05 percent bymass and more preferably less than 0.045 percent by mass.

Nitrogen Content of Less than 0.09 Percent by Mass

Nitrogen as well as carbon is an element useful in enhancing thehardness of the steel by hardening. In particular, nitrogen forms fineCr₂N precipitates at 500° C. to 700° C.; hence, the temper softeningresistance is improved by the effect of hardening due to precipitation.Therefore, the use of nitrogen rather than carbon is effective inenhancing the temper softening resistance. In order to achieve such anadvantage, the steel preferably contains 0.015 percent or more nitrogenon a mass basis. In order to further enhance the temper softeningresistance, the nitrogen content is more than 0.03 percent by mass. Onthe other hand, an excessive increase in nitrogen content leads to areduction in toughness; hence, the nitrogen content must be limited toless than 0.09 percent by mass. Silicon content of 1.0 percent by massor less

Since an excessive increase in silicon content leads to a reduction intoughness, the silicon content is limited to 1.0 percent by mass orless. The silicon content is preferably 0.3 percent by mass or less.

Manganese Content of 2.0 Percent by Mass or Less

Manganese bonds to sulfur to form MnS, which causes a reduction incorrosion resistance. Therefore, the manganese content is limited to 2.0percent by mass or less. The manganese content is preferably less than1.0 percent by mass and more preferably less than 0.5 percent by mass.

Phosphorus Content of 0.04 Percent by Mass or Less

Phosphorus is an element that causes a reduction in hot workability.Therefore, in view of production, it is preferable that the phosphoruscontent be minimized. However, since an excessive reduction inphosphorus content leads to an increase in steel making cost, the upperlimit of the phosphorus content is 0.04 percent by mass. In view of hotworkability, the phosphorus content is preferably 0.02 percent by massor less.

Sulfur Content of 0.010 Percent by Mass or Less

Since an increase in sulfur content leads to a reduction in hotworkability, the sulfur content as well as the phosphorus content ispreferably low. In consideration of the desulfurization cost in steelmaking steps, the sulfur content is 0.010,percent by mass or less. Inview of hot workability, the sulfur content is preferably 0.005 percentby mass or less.

Aluminum Content of 0.2 Percent by Mass or Less

Since an excessive increase in aluminum content leads to a reduction intoughness, the aluminum content is limited to 0.2 percent by mass orless. The aluminum content is preferably 0.20 percent by mass or lessand more preferably 0.05 percent by mass or less.

Chromium Content of More than 11.5 to 15.0 Percent by Mass

Chromium is an element essential to achieve corrosion resistance whichis an advantage of the stainless steel. In order to achieve highcorrosion resistance, it is necessary that the chromium content be morethan 11.5 percent by mass. In order to achieve higher corrosionresistance, the chromium content is preferably 12.0 percent by mass ormore. On the other hand, an increase in chromium content leads to areduction in workability and a reduction in toughness. In particular,when the chromium content is more than 15.0 percent by mass, thestainless steel has seriously low toughness. Therefore, the upper limitof the chromium content is limited to 15.0 percent by mass. In order toachieve high toughness, the chromium content is preferably less than14.0 percent by mass and more preferably less than 13.0 percent by mass.

Nickel Content of 0.5 to 2.0 Percent by Mass

Nickel inhibits chromium carbonitride from precipitating at hightemperature more than 600° C. to maintain the hardness of martensiticstructures supersaturatedly containing dissolved carbon, therebyenhancing the temper softening resistance. Furthermore, nickel is usefulin enhancing the corrosion resistance which is an advantage of thestainless steel and also useful in enhancing the toughness. In order toachieve such advantages, it is necessary that the nickel content be 0.5percent by mass or more. The nickel content is preferably more than 0.50percent by mass and more preferably 0.55 percent by mass or more.Furthermore, in order to achieve high temper softening resistance, thenickel content is preferably more than 1.0 percent by mass. On the otherhand, when the nickel content is more than 2.0 percent by mass, theincrease in temper softening resistance is saturated and raw materialcost is increased. Therefore, the nickel content is limited to 2.0percent by mass or less.

Copper Content of More than 0.50 to 4.0 Percent by Mass

Copper forms fine ε-Cu precipitates at about 600° C. and the tempersoftening resistance is improved by the effect of the precipitates. Inorder to achieve such an advantage, the copper content is more than 0.50percent by mass and preferably more than 0.5 percent by mass. In orderto achieve high temper softening resistance, the copper content ispreferably 1.0 percent by mass or more and more preferably 1.5 percentby mass or more. On the other hand, when the copper content is more than4.0 percent by mass, the temper softening resistance is saturated andraw material cost is increased. Therefore, the copper content is limitedto 4.0 percent by mass or less.

Niobium Content of More than 0.08 to 0.6 Percent by Mass

Niobium strongly bonds to carbon or nitrogen to form niobium carbideprecipitates or niobium nitride precipitates, respectively. Theseprecipitates have no influence on the hardness of the hardened stainlesssteel but inhibit a strain introduced into martensitic structures byhardening from being relieved; thereby enhancing the temper softeningresistance of the stainless steel maintained at about 600° C. In orderto achieve such an advantage, it is necessary that the niobium contentbe more than 0.08 percent by mass. The niobium content is preferably0.10 percent by mass or more. On the other hand, when the niobiumcontent is more than 0.6 percent by mass, the temper softeningresistance is saturated and the toughness is reduced; hence, the niobiumcontent is limited to 0.6 percent by mass or less. In view of toughness,the niobium content is preferably 0.4 percent by mass or less and morepreferably 0.2 percent by mass or less.

Inequality 0.03≦[C]+[N]−13/93×[Nb]≦0.09   (1)

Carbon and nitrogen are elements essential to enhance the hardness ofthe steel by hardening. However, niobium carbide produced by thereaction between carbon and niobium and niobium nitride produced by thereaction between nitrogen and niobium are useless in increasing thehardness. Therefore, in order to control the hardness of the steel byhardening, the effects of carbon and nitrogen must be estimated usingthe middle term [C]+[N]−13/93×[Nb] of inequality (1), wherein the middleterm represents the remainder obtained by subtracting the carbon andnitrogen content of the precipitates from that of the steel. When themiddle term is less than 0.03, the hardness is less than 32 in HRC. Incontrast, when the middle term is more than 0.09, the hardness is morethan 38 in HRC. Therefore, in order to allow the hardened steel to havea hardness of 32 to 38 in HRC, which is suitable for disc brake uses,the middle term of inequality (1) is limited to 0.03 to 0.09.

Inequality5×[Cr]+10×[Si]+30×[Nb]−9×[Ni]−5×[Mn]−3×[Cu]−225×[N]−270×[C]≦40   (2)

Inequality (2) is useful in evaluating the hardenability. In order toallow a disc material to have high hardenability, it is necessary that90 percent or more austenite be formed on a volume basis by heating thedisc material at 900° C. to 1000° C. and transformed into martensite byair-cooling the heated disc material. In ordinary martensitic stainlesssteels, the amount of austenite is maximized at about 1000° C. anddecreased at higher than or lower than 1000° C. When the left side ofinequality (2) is more than 40, a temperature range in which 90 percentor more austenite is formed on a volume basis is small. Hence, the steelcan hardly be sufficiently hardened, that is, the hardness of the steelis outside a proper hardness range if the hardening temperature isfluctuated in a manufacturing step. In view of productivity, thehardening temperature is preferably low because an increase in hardeningtemperature increases heating cost and heating time. From this view, itis critical that the steel hardened from 900° C. have a hardness of 32or more in HRC. Therefore, in order to achieve high hardenability, it isnecessary that the left side of inequality (2) be limited to 40 or less.

In the present invention, the steel preferably contains components belowin addition to the above essential elements. Vanadium content of 0.02 to0.3 percent by mass

Vanadium as well as niobium is an element useful in forming finecarbonitride precipitates to enhance the temper softening resistance.The vanadium content is preferably 0.02 percent by mass or more and morepreferably 0.10 percent by mass or more. However, when the vanadiumcontent is more than 0.3 percent by mass, the toughness is low.Therefore, the upper limit of the vanadium content is preferably 0.3percent by mass.

One or Both of Molybdenum Content of 0.02 to 2.0 Percent by Mass andCobalt Content of 0.02 to 2.0 Percent by Mass

Molybdenum and cobalt are elements useful in enhance the corrosionresistance; hence, the steel may 0.02 percent or more molybdenum and/or0.02 percent or more cobalt on a mass basis as needed. Molybdenum aswell as nickel inhibits chromium carbonitride from precipitating tomaintain the hardness of martensitic structures supersaturatedlycontaining carbon, thereby enhancing the temper softening resistance. Inorder to enhance the corrosion resistance, the steel preferably contains0.5 percent or more of each element on a mass basis. When the molybdenumcontent and the cobalt content are 1.5 percent by mass or lessrespectively, the steel has sufficiently high corrosion resistance. Incontrast, when the molybdenum content and the cobalt content are morethan 2.0 percent by mass respectively, the effect of improving thecorrosion resistance is saturated and the toughness is decreased.Therefore, the upper limit of the molybdenum content and that of thecobalt content are preferably 2.0 percent by mass respectively.

One or More of Titanium Content of 0.02 to 0.3 Percent by Mass,Zirconium Content of 0.02 to 0.3 Percent by Mass, and Tantalum Contentof 0.02 to 0.3 Percent by Mass

Titanium, zirconium, and tantalum as well as niobium are elements usefulin creating fine carbonitride precipitates to enhance the tempersoftening resistance. The steel may contain 0.02 percent or more of eachelement on a mass basis as needed. However, when the content of theelement is more than 0.3 percent by mass, the toughness is low.Therefore, the upper limit of the content of the element is preferably0.3 percent by mass.

One or Both of Boron Content of 0.0005 to 0.0050 Percent by Mass andCalcium Content of 0.0005 to 0.0050 Percent by Mass

Boron and calcium are useful in enhancing the toughness of the steel ifthe boron or calcium content is small. Hence, the steel preferablycontains 0.0005 percent or more boron and/or 0.0005 percent or morecalcium on a mass basis. However, when the boron or calcium content ismore than 0.0050 percent by mass respectively, the effect is saturatedand the corrosion resistance is decreased. Therefore, the upper limit ofthe boron or calcium content is preferably 0.0050 percent by massrespectively.

The martensitic stainless steel of the present invention contains ironand avoidable impurities in addition to the above components. Examplesof the impurities (0.01 percent or less by mass) include alkali metals,alkaline-earth metals, rare-earth element, and transition metals, forexample, sodium, barium, lanthanum, yttrium, and hafnium. The impuritiesdo not reduce the advantages of the present invention.

The structure of a sheet made of the martensitic stainless steel fordisc brakes according to the present invention will now be described.

In the steel of the present invention, in order to achieve sufficientlyhigh hardenability, it is necessary that 90 percent or more austenite beformed on a volume basis by heating the steel at 900° C. to 1000° C. andtransformed into martensite by air-cooling the heated steel. Therefore,the steel preferably contains 90 percent or more martensite on a volumebasis, the remainder being ferrite. When the volume of martensite isless than 90%, the volume of ferrite, which is soft, is large; hence,desired hardness can hardly be achieved.

A method for manufacturing the martensitic stainless steel according tothe present invention will now be described.

The method for manufacturing the steel is not particularly limited andany known method for manufacturing an ordinary martensitic stainlesssteel may be used.

The method preferably includes a step of preparing molten steelcontaining the above essential components and additional components usedas needed in a converter or an electric furnace and a step of subjectingthe molten steel to secondary smelting by a smelting process such as avacuum degassing process (an RH process), VOD (vacuum oxygendecarburization), or AOD (argon oxygen decarburization). The moltensteel may be formed into a steel material (a slab) by a known processsuch as a continuous casting process or a slabbing process. In view ofproductivity and quality, the continuous casting process is preferablyused. The obtained steel material is heated at 1100° C. to 1250° C.,hot-rolled at a finishing temperature of 800° C. to 1100° C., and thencoiled at 600° C. to 900° C., whereby a hot-rolled steel strip with athickness of 3 to 8 mm is prepared. The hot-rolled steel strip isannealed at 650° C. to 900° C. for four to 20 hours in a batch-typefurnace such as a box annealing furnace and then rolled into a sheet asneeded, whereby a disc material is prepared. The hot-rolled steel stripmay be descaled by pickling or shot blast.

The disc material obtained as described above is stamped into pieceshaving a disc shape. Each piece is heated at 900° C. to 1000° C.,hardened by an air-cooling process or another process by which the piececan be cooled at a cooling rate of air-cooling or higher, and thensubjected to descaling and/or coating as needed. Friction faces of theresulting piece that are rubbed with brake pads are mechanicallypolished such that the piece is improved in beauty and thicknessaccuracy, whereby a disc product having a configuration shown in FIG. 1is obtained.

The martensitic stainless steel, manufactured as described above,according to the present invention can be used to manufacture brakediscs for motorcycles, bicycles, motorcars, and snow mobiles. Sincediscs for bicycle disc brakes have a thickness of about 2 mm, thehot-rolled steel strip annealed and then pickled is cold-rolled with atandem mill or a reverse mill such as a Sendzimir mill, annealed at 600°C. to 900° C. as needed, and then pickled as needed, whereby a materialfor such discs is prepared. This disc material can be processed intoproducts by the same procedure as that for manufacturing the formerproduct using the hot-rolled steel strip.

EXAMPLE 1

Steel samples (Nos. 1 to 67) containing chemical components shown inTables 1 to 4 were prepared in a small-sized vacuum melting furnace.Each steel sample was cast into an ingot with a weight of 50 kgf. Theingot was hot-rolled into a sheet with a thickness of 5 mm at afinishing temperature of 900° C. The hot-rolled sheet was annealed at700° C. for eight hours in an argon atmosphere, gradually cooled, andthen pickled in such a manner that the sheet was immersed in an acidmixture (an aqueous solution containing 10 mass % nitric acid and 3 mass% hydrofluoric acid) maintained at 60° C. such that scale on the sheetis removed, whereby a test specimen investigated as described below wasprepared.

(Hardenability)

Two test pieces were prepared by cutting each test specimen. The testpieces were 30 mm square and had a thickness equal to the sheetthickness. One of the pieces was heated at 900° C. for ten minutes andthe other one was heated at 1000° C. for ten minutes. The resultingpieces were hardened by air-cooling. Scale was removed from surfaces ofthe pieces by pickling. The surface hardness in HRC (JIS Z 2245) wasmeasured at five points per test piece and obtained measurements wereaveraged. It is necessary for steel for discs to have a hardness of 32to 38 in HRC after the steel is hardened at 900° C. or 1000° C. asdescribed above. Hence, if one of the test pieces hardened at the abovetemperature has a hardness outside the above range, the test specimencan be evaluated to be inferior in stability of hardenability; that is,there is a possibility that the test specimen has an insufficienthardness due to fluctuations in heat-treating temperature.

(Temper Softening Resistance)

Other two test pieces were prepared by cutting each test specimen. Thesetest pieces were 30 mm square and had a thickness equal to the sheetthickness. The test pieces were heated at 1000° C. for ten minutes andthen hardened by air-cooling. One of the test pieces was tempered at650° C. for one hour and the other one was tempered at 670° C. for onehour. Scale was removed from surfaces of the pieces by pickling. Thesurface hardness (HRC) was measured at five points per test piece andobtained measurements were averaged, whereby the temper softeningresistance of the test pieces tempered at 650° C. and 670° C. wasevaluated. If the test piece tempered at 650° C. has a hardness of 32 ormore in HRC and the test piece tempered at 670° C. has a hardness of 30or more in HRC, the test specimen can be evaluated to have sufficientlyhigh temper softening resistance.

(Corrosion Resistance)

A corrosion resistance test was performed as follows: each test specimenwas heated at 1000° C. for ten minutes, hardened by air-cooling, andthen tempered at 650° C. for one hour; a test piece having a width of.70 mm, a length of 150 mm, and a thickness equal to the sheet thicknesswas prepared by cutting the resulting test specimen; a test face (a testface of the test piece) was wet-polished with a sheet of #800 emerypolishing paper; the resulting test piece was subjected to a salt spraytest for eight hours according to JIS Z 2371; and the number of rustspots was counted. The corrosion resistance was evaluated as follows: arating of ◯ was given to the test pieces having no rust spot, a ratingof Δ was given to the test pieces having one to four rust spots, and arating of × was given to the test pieces having five or more rust spots.The test pieces having five or more rust spots can be evaluated to beinferior in corrosion resistance and are not therefore suitable forpractical use.

(Toughness)

The toughness was measured as follows: each test specimen was heated at1000° C. for ten minutes, hardened by air-cooling, and then tempered at650° C. for one hour; three subsize Charpy impact test pieces (athickness of 10 mm, a width of 5 mm (equal to the thickness of thehot-rolled sheet), and a length of 55 mm) were prepared by cutting theresulting test specimen according to JIS Z 2202; the Charpy impact valueof the test pieces was measured by performing the Charpy impact test at25° C. (JIS Z 2242); and obtained measurements were then averaged. Ifthe test specimen has an average Charpy impact value of 50 J/cm² ormore, the specimen can be evaluated to be suitable for practical use.

Tables 1 to 4 show the results of the above tests. The steel samples(Nos. 1 to 49) shown in Tables 1, 2, and 3 comply with the standards ofthe present invention. For the steel samples, the test pieces hardenedfrom 900° C. and 1000° C. have a proper hardness, that is, a hardness of32 to 38 in HRC. The test pieces hardened from 1000° C. and thentempered at 650° C. have a hardness of 32 or more in HRC. The testpieces hardened from 1000° C. and then tempered at 670° C. have ahardness of 30 or more in HRC. The test pieces subjected to the impacttest have a Charpy impact value of 50 J/cm² or more. The test piecessubjected to the salt spray test have high corrosion resistance. Incontrast, the steel samples (Nos. 50 to 67) shown in Tables 4 do notcomply with the standards of the present invention. For these steelsamples, the test pieces have low hardness, low Charpy impact value,and/or low corrosion resistance after they have been hardened from 900°C. or 1000° C. or tempered at 650° C. or 670° C. As is clear from theabove results, the hot-rolled sheets prepared using the steel sampleshaving the same composition as that of the martensitic stainless steelof the present invention have satisfactory properties and are suitablefor disc brakes.

EXAMPLE 2

Properties of a cold-rolled steel sheet were investigated. An annealedcold-rolled sheet was obtained by the following procedure: the testspecimen prepared by processing the annealed hot-rolled sheet, preparedusing Steel Sample 1 shown in Table 1 of Example 1, having a thicknessof 5 mm was cold-rolled, whereby a cold-rolled sheet with a thickness of1.5 mm was prepared; the cold-rolled sheet was annealed in such a mannerthat the sheet was heated at 750° C. for one minute and then air-cooled;the resulting cold-rolled sheet was descaled in such a manner that thesheet was immersed in an acid mixture (10% nitric acid and 3%hydrofluoric acid on a mass basis) maintained at 60° C. The annealedcold-rolled sheet was tested in the same manner as that described inExample 1. A subsize Charpy impact test piece had a width of 1.5 mm (thethickness of the cold-rolled sheet). Results obtained by testing thecold-rolled sheet were as follows: test pieces hardened from 900° C. hada hardness of 37 in HRC, test pieces hardened from 1000° C. had ahardness of 37 in HRC, test pieces hardened from 1000° C. and thentempered at 650° C. had a hardness of 34 in HRC, and test pieceshardened from 1000° C. and then tempered at 670° C. had a hardness of 32in HRC. The subsize Charpy impact test piece had a Charpy impact valueof 85 J/cm². Test pieces subjected to the salt spray test had no rustspot; that is, these test pieces had high corrosion resistance. As isclear from these results, the cold-rolled steel sheet prepared using thesteel sample having the same composition as that of the martensiticstainless steel of the present invention has satisfactory properties andare suitable for disc brakes.

INDUSTRIAL APPLICABILITY

As described above, according to the present invention, a martensiticstainless steel, having high temper softening resistance, for discbrakes can be manufactured by properly controlling the compositionthereof. In particular, the martensitic stainless steel is not seriouslysoftened by maintaining the steel at more than 600° C. The steel has ahardness of 32 or more or a hardness of 30 or more in HRC after thesteel is tempered at 650° C. for one hour or tempered at 670° C. for onehour, respectively.

TABLE 1 Composition (percent by mass) Steel Other Samples C Si Mn P S AlCr Ni Cu Nb N Elements  1 0.041 0.24 0.33 0.01 0.004 0.012 12.2 1.211.65 0.17 0.057 —  2 0.048 0.23 0.18 0.02 0.003 0.003 12.7 1.18 1.620.21 0.042 —  3 0.042 0.23 0.32 0.02 0.003 0.013 12.2 1.20 0.58 0.180.053 —  4 0.044 0.22 0.44 0.03 0.001 0.003 12.5 1.32 1.14 0.18 0.043 — 5 0.018 0.86 0.38 0.02 0.004 0.013 12.3 1.58 1.58 0.18 0.053 —  6 0.0430.24 0.43 0.02 0.003 0.003 12.9 1.77 1.61 0.09 0.055 —  7 0.011 0.210.34 0.02 0.003 0.002 12.4 1.12 1.92 0.19 0.085 —  8 0.043 0.18 0.780.02 0.005 0.003 12.7 1.68 1.62 0.15 0.045 Ti: 0.15  9 0.043 0.17 0.350.01 0.002 0.008 14.4 1.58 1.75 0.38 0.071 — 10 0.041 0.19 1.75 0.020.003 0.003 12.3 1.57 1.60 0.18 0.036 — 11 0.033 0.12 0.42 0.02 0.0040.015 11.8 1.51 3.65 0.23 0.046 — 12 0.042 0.21 0.42 0.02 0.003 0.00312.3 0.57 2.04 0.16 0.043 — 13 0.037 0.29 0.41 0.02 0.004 0.013 12.51.17 1.85 0.55 0.082 — 14 0.042 0.22 0.42 0.01 0.003 0.005 12.3 1.072.05 0.19 0.024 — 15 0.042 0.23 0.25 0.02 0.003 0.018 12.3 1.88 1.500.33 0.063 Mo: 0.85 16 0.033 0.15 1.55 0.02 0.003 0.003 12.3 0.47 1.080.15 0.045 — 17 0.037 0.23 0.45 0.02 0.004 0.016 12.8 1.94 1.55 0.250.035 Mo: 1.43 18 0.039 0.22 0.33 0.02 0.003 0.023 12.9 1.17 1.62 0.150.048 Mo: 1.05 19 0.044 0.21 0.25 0.02 0.003 0.017 12.2 1.10 2.12 0.230.054 Mo: 0.12 20 0.043 0.15 0.18 0.02 0.003 0.022 12.1 1.13 2.07 0.390.052 Mo: 1.75 21 0.039 0.23 0.35 0.02 0.003 0.002 12.2 1.22 1.61 0.170.055 V: 0.05 22 0.041 0.23 0.33 0.02 0.002 0.002 12.3 1.22 0.59 0.180.053 V: 0.13 23 0.042 0.23 0.34 0.02 0.003 0.003 12.8 1.47 1.14 0.090.044 V: 0.12 24 0.040 0.18 1.55 0.02 0.002 0.003 12.2 0.51 1.11 0.180.035 V: 0.13 25 0.041 0.23 0.35 0.02 0.002 0.003 12.4 0.46 2.54 0.160.044 V: 0.19 26 0.035 0.30 0.45 0.03 0.004 0.003 12.4 1.15 1.55 0.550.085 V: 0.30 Value of Value of Hardness of Hardened Hardness ofHardness of Middle Left Side Test Pieces (HRC) Test Pieces Test PiecesCharpy Term of of Quenching Quenching Temperature Temperature SaltImpact Steel Inequality Inequality Temperature Temperature at 650° C. at670° C. Spray Value Samples (1) (2) at 900° C. at 1000° C. (HRC) (HRC)Test (J/cm²) Remarks  1 0.074 27 37 37 35 32 ○ 78 Example  2 0.061 33 3435 33 31 Δ 58 Example  3 0.070 31 35 36 32 30 ○ 68 Example  4 0.062 3135 35 33 30 ○ 73 Example  5 0.046 38 33 34 32 30 ○ 56 Example  6 0.08523 37 37 32 30 ○ 74 Example  7 0.069 30 34 35 33 31 ○ 73 Example  80.067 24 34 35 33 31 Δ 68 Example  9 0.061 36 33 35 32 30 ○ 52 Example10 0.052 22 34 35 33 30 Δ 83 Example 11 0.047 21 33 33 32 30 Δ 72Example 12 0.063 34 35 35 32 30 ○ 79 Example 13 0.042 35 33 34 32 30 ○52 Example 14 0.039 35 33 34 32 30 ○ 86 Example 15 0.059 26 34 34 33 31○ 73 Example 16 0.06  33 35 35 32 30 ○ 70 Example 17 0.037 32 33 33 3230 ○ 74 Example 18 0.066 33 33 35 32 30 ○ 75 Example 19 0.066 28 34 3433 31 ○ 75 Example 20 0.040 33 33 33 32 30 ○ 81 Example 21 0.070 28 3636 34 32 ○ 80 Example 22 0.069 32 35 36 32 30 ○ 70 Example 23 0.073 2936 36 32 30 ○ 75 Example 24 0.050 34 35 35 33 31 Δ 65 Example 25 0.06335 34 34 32 30 ○ 53 Example 26 0.043 38 33 34 32 30 ○ 52 Example

TABLE 2 Composition (percent by mass) Steel Other Samples C Si Mn P S AlCr Ni Cu Nb N Elements 27 0.042 0.28 0.12 0.02 0.003 0.003 11.8 1.121.73 0.22 0.066 V: 0.12, B: 0.0015 28 0.048 0.25 0.18 0.02 0.003 0.00312.8 1.15 1.62 0.23 0.043 — 29 0.043 0.23 0.34 0.02 0.003 0.023 12.11.20 1.11 0.18 0.054 Co: 0.12, B: 0.0009 30 0.044 0.31 0.44 0.03 0.0010.003 12.5 0.52 1.04 0.15 0.043 Co: 0.30 31 0.038 0.16 0.28 0.02 0.0040.003 12.2 0.58 1.68 0.18 0.043 Ca: 0.0011 32 0.043 0.21 0.43 0.02 0.0030.003 12.9 1.87 1.61 0.09 0.055 Co: 1.2, Ca: 0.0006 33 0.012 0.21 0.330.02 0.003 0.012 12.5 1.12 1.82 0.19 0.084 Mo: 0.8, V: 0.05 34 0.0430.15 0.28 0.02 0.005 0.003 12.7 1.66 1.62 0.16 0.015 B: 0.0045 35 0.0420.17 0.34 0.01 0.002 0.003 12.8 1.08 1.64 0.58 0.071 Mo: 0.7, V: 0.08 360.041 0.09 1.55 0.02 0.003 0.003 12.3 0.54 1.10 0.15 0.041 Ca: 0.0046 370.036 0.12 0.43 0.02 0.002 0.003 12.1 1.51 1.65 0.23 0.046 Co: 0.8, Ta:0.08 38 0.042 0.29 0.42 0.02 0.003 0.003 12.2 1.07 2.04 0.16 0.042 Mo:1.5, V: 0.12 39 0.018 0.29 0.43 0.02 0.004 0.003 12.5 0.97 1.95 0.150.082 Co: 0.03, B: 0.0019 Value of Value of Hardness of HardenedHardness of Hardness of Middle Left Side Test Pieces (HRC) Test PiecesTest Pieces Charpy Term of of Quenching Quenching TemperatureTemperature Salt Impact Steel Inequality Inequality TemperatureTemperature at 650° C. at 670° C. Spray Value Samples (1) (2) at 900° C.at 1000° C. (HRC) (HRC) Test (J/cm²) Remarks 27 0.077 26 37 37 34 32 Δ68 Example 28 0.058 35 33 35 33 30 Δ 55 Example 29 0.072 29 35 36 33 31○ 67 Example 30 0.66 39 33 35 33 30 ○ 53 Example 31 0.056 36 33 35 33 31○ 66 Example 32 0.085 22 37 37 33 30 ○ 74 Example 33 0.069 31 34 35 3432 ○ 63 Example 34 0.036 34 33 34 33 30 ○ 68 Example 35 0.032 39 32 3434 32 ○ 54 Example 36 0.061 31 34 35 32 30 Δ 64 Example 37 0.050 28 3535 34 31 ○ 62 Example 38 0.062 30 35 35 34 32 ○ 69 Example 39 0.079 3037 37 32 30 ○ 72 Example

TABLE 3 Composition (percent by mass) Steel Other Samples C Si Mn P S AlCr Ni Cu Nb N Elements 40 0.042 0.25 0.42 0.02 0.003 0.005 12.4 1.072.05 0.16 0.044 Mo: 1.1, V: 0.28 41 0.041 0.91 0.15 0.02 0.003 0.00312.3 1.88 1.60 0.33 0.063 Mo: 0.2, Ti: 0.05 42 0.036 0.12 0.12 0.020.002 0.003 12.1 1.12 1.88 0.25 0.048 Mo: 1.9, V: 0.04 43 0.036 0.230.33 0.04 0.008 0.003 12.8 1.94 1.52 0.25 0.038 Mo: 0.7, Zr: 0.14 440.039 0.23 1.87 0.02 0.003 0.023 12.9 1.11 1.62 0.26 0.045 V: 0.2, Ca:0.0012 45 0.043 0.21 0.28 0.01 0.003 0.003 12.2 1.10 2.03 0.47 0.054 V:0.11 46 0.043 0.12 0.18 0.02 0.002 0.152 12.1 1.12 2.07 0.48 0.056 Mo:1.3, V: 0.03 47 0.041 0.26 0.65 0.02 0.003 0.022 12.3 1.06 1.85 0.220.047 V: 0.14, B: 0.0012 48 0.043 0.12 0.31 0.02 0.002 0.003 12.1 1.710.58 0.25 0.058 Co: 0.08, Ca: 0.0036 49 0.044 0.30 0.05 0.03 0.005 0.00314.3 1.1 20.05 0.15 0.055 Mo: 0.7, V: 0.13 Value of Value of Hardness ofHardened Hardness of Hardness of Middle Left Side Test Pieces (HRC) TestPieces Test Pieces Charpy Term of of Quenching Quenching TemperatureTemperature Salt Impact Steel Inequality Inequality TemperatureTemperature at 650° C. at 670° C. Spray Value Samples (1) (2) at 900° C.at 1000° C. (HRC) (HRC) Test (J/cm²) Remarks 40 0.064 30 35 35 34 32 ○63 Example 41 0.058 33 34 35 34 32 ○ 53 Example 42 0.049 32 35 35 34 32○ 74 Example 43 0.039 32 33 34 34 32 ○ 69 Example 44 0.048 30 33 34 3432 Δ 71 Example 45 0.031 36 32 34 34 32 ○ 50 Example 46 0.032 35 32 3434 32 ○ 53 Example 47 0.057 31 34 35 34 32 Δ 72 Example 48 0.066 26 3535 32 30 ○ 68 Example 49 0.078 38 35 37 34 32 ○ 55 Example

TABLE 4 Composition (percent by mass) Steel Other Samples C Si Mn P S AlCr Ni Cu Nb N Elements 50 0.051 0.14 0.12 0.02 0.003 0.003 12.2 1.052.03 0.15 0.043 — 51 0.033 1.12 0.16 0.02 0.002 0.005 12.3 1.68 1.880.30 0.065 Mo: 0.7, Zr: 0.12 52 0.043 0.21 2.12 0.02 0.003 0.006 12.11.22 1.80 0.43 0.064 Ti: 0.25 53 0.049 0.13 1.78 0.03 0.008 0.005 12.20.10 1.23 0.26 0.018 — 54 0.080 0.02 0.25 0.02 0.005 0.031 12.2 0.850.35 0.04 0.011 — 55 0.050 0.25 2.12 0.02 0.003 0.001 12.5 0.86 0.010.25 0.030 — 56 0.046 0.32 1.52 0.02 0.002 0.007 12.3 0.27 0.01 0.130.037 — 57 0.040 0.12 0.31 0.03 0.002 0.022 11.2 1.43 1.88 0.23 0.055Ta: 0.12 58 0.043 0.23 0.44 0.02 0.002 0.009 12.2 0.44 3.21 0.25 0.055 —59 0.043 0.29 0.43 0.03 0.003 0.009 12.4 1.06 1.89 0.62 0.084 V: 0.06 600.043 0.19 0.34 0.01 0.003 0.005 12.5 1.72 0.41 0.19 0.059 Co: 0.08, Ca:0.0023 61 0.008 0.16 0.21 0.02 0.002 0.006 13.3 1.14 1.63 0.00 0.092 —62 0.023 0.14 0.19 0.02 0.005 0.221 12.2 1.65 2.23 0.16 0.076 — 63 0.0430.12 0.34 0.02 0.002 0.012 12.1 1.18 1.56 0.08 0.035 — 64 0.033 0.230.34 0.01 0.003 0.003 12.1 1.46 1.66 0.34 0.043 B: 0.0025 65 0.038 0.130.08 0.03 0.002 0.013 12.8 1.08 1.58 0.16 0.077 Mo: 1.3, V: 0.03 660.033 0.21 0.12 0.02 0.003 0.005 12.8 1.13 1.54 0.31 0.044 Mo: 1.9, V:0.04 67 0.039 0.22 0.33 0.02 0.002 0.033 15.4 1.93 1.51 0.20 0.058 —Value of Value of Hardness of Hardened Hardness of Hardness of MiddleLeft Side Test Pieces (HRC) Test Pieces Test Pieces Charpy Term of ofQuenching Quenching Temperature Temperature Salt Impact Steel InequalityInequality Temperature Temperature at 650° C. at 670° C. Spray ValueSamples (1) (2) at 900° C. at 1000° C. (HRC) (HRC) Test (J/cm²) Remarks50 0.073 27 35 35 33 31 X 15 Comparative Example 51 0.056 37 32 35 33 31○ 23 Comparative Example 52 0.047 23 34 34 32 30 X 65 ComparativeExample 53 0.031 39 32 33 24 21 X 16 Comparative Example 54 0.085 28 3737 22 19 X 14 Comparative Example 55 0.045 34 32 34 24 21 X 22Comparative Example 56 0.065 38 33 35 21 18 X 19 Comparative Example 570.063 21 35 35 33 31 X 65 Comparative Example 58 0.063 31 34 35 28 25 X29 Comparative Example 59 0.040 36 32 34 32 30 ○ 23 Comparative Example60 0.075 27 36 36 26 23 ○ 69 Comparative Example 61 0.100 29 39 40 20 16○ 18 Comparative Example 62 0.077 21 36 36 33 31 ○ 16 ComparativeExample 63 0.067 28 35 35 23 20 ○ 69 Comparative Example 64 0.028 35 2829 25 22 ○ 65 Comparative Example 65 0.093 28 39 40 38 35 ○ 64Comparative Example 66 0.034 42 30 33 32 30 ○ 69 Comparative Example 670.069 38 33 36 32 30 ○ 25 Comparative Example Note: Underlined valuesare outside the scope of the present invention.

1. A brake disc for a disc brake system, the brake disc being in a discshape and made of a martensitic stainless steel, the steel comprising:less than 0.050 mass % carbon, 1.0 mass % or less silicon, 2.0 mass % orless manganese, 0.04 mass % or less phosphorus, 0.010 mass % or lesssulfur, 0.2 mass % or less aluminum, chromium in a range of from morethan 11.5 mass % to 15.0 mass %, nickel in a range of from 0.5 mass % to2.0 mass %, copper in a range of from more than 0.50 mass % to 4.0 mass%, niobium in a range of from more than 0.08 mass % to 0.6 mass %, lessthan 0.09 mass % nitrogen, and 0.0005 mass % to 0.0050 mass % calcium,the remainder being iron and unavoidable impurities, wherein the carboncontent, the nitrogen content, the niobium content, the chromiumcontent, the silicon content, the nickel content, the manganese content,and the copper content satisfy the following relationships (1) and (2):0.03≦[C]+[N]−13/93×[Nb]≦0.09   (1)5×[Cr]+10×[Si]+30×[Nb]−9×[Ni]−5×[Mn]−3×[Cu]−225×[N]−270×[C]≦40   (2),the steel sheet being 90 percent or more martensite on a volume basis,and the remainder of the volume of the steel being ferrite.
 2. The brakedisc according to claim 1, further containing 0.02 mass % to 0.3 mass %vanadium.
 3. The brake disc according to claim 1, further containing oneor both of 0.02 mass % to 2.0 mass % molybdenum and 0.02 mass % to 2.0mass % cobalt.
 4. The brake disc according to claim 1, furthercontaining one or more of 0.02 mass % to 0.3 mass % titanium, 0.02 mass% to 0.3 mass % zirconium, and 0.02 mass % to 0.3 mass % tantalum. 5.The brake disc according to claim 1, further containing 0.0005 mass % to0.0050 mass % boron.
 6. The brake disc according to claim 2, furthercontaining one or both of 0.02 mass % to 2.0 mass % molybdenum and 0.02mass % to 2.0 mass % cobalt.
 7. The brake disc according to claim 2,further containing one or more of 0.02 mass % to 0.3 mass % titanium,0.02 mass % to 0.3 mass % zirconium, and 0.02 mass % to 0.3 mass %tantalum.
 8. The brake disc according to claim 3, further containing oneor more of 0.02 mass % to 0.3 mass % titanium, 0.02 mass % to 0.3 mass %zirconium, and 0.02 mass % to 0.3 mass % tantalum.
 9. The brake discaccording to claim 6, further containing one or more of 0.02 mass % to0.3 mass % titanium, 0.02 mass % to 0.3 mass % zirconium, and 0.02 mass% to 0.3 mass % tantalum.
 10. The brake disc according to claim 2,further containing 0.0005 mass % to 0.0050 mass % boron.
 11. The brakedisc according to claim 3, further containing 0.0005 mass % to 0.0050mass % boron.
 12. The brake disc according to claim 4, furthercontaining 0.0005 mass % to 0.0050 mass % boron.
 13. The brake discaccording to claim 6, further containing 0.0005 mass % to 0.0050 mass %boron.
 14. The brake disc according to claim 7, further containing0.0005 mass % to 0.0050 mass % boron.
 15. The brake disc according toclaim 8, further containing 0.0005 mass % to 0.0050 mass % boron. 16.The brake disc according to claim 9, further containing 0.0005 mass %.to0.0050 mass % boron.
 17. The brake disc according to claim 1, whereinthe martensitic stainless steel is hot-rolled.
 18. The brake discaccording to claim 1, wherein the martensitic stainless steel iscold-rolled.
 19. The brake disc according to claim 1, wherein themartensitic stainless steel has a hardness of 32 to 38 in HRC.
 20. Thebrake disc according to claim 1, wherein the brake disc comprises acircular opening in a center portion of the brake disc, the circularopening being surrounded by a first circular portion having a pluralityof fixing holes, the first circular portion being surrounded by a secondcircular portion having a plurality of cooling holes, the secondcircular portion forming an outer circumference of the brake disc. 21.The brake disc according to claim 20, wherein the second circularportion having the plurality of cooling holes is a friction section.